Wax-based compositions, such as moistureproof coatings, have been prepared mostly by mechanically mixing constituent components in their molten state (see JP-A-7-278510) or mixing a wax emulsion with other component (see JP-A-59-66598).
However, the above-described techniques have the following problems. When a solid or high-viscous substance is mixed into a molten wax by mechanical mixing, it has been difficult to uniformly disperse the substance because the viscosity of the molten wax is so low that sufficient shear force is not exerted on the substance to be mixed up. In using a wax emulsion, it is impossible to disperse a component whose particle size is equal to or smaller than that of the component constituting the emulsion, which has made it difficult to obtain a uniform disperse system.
A wax is widely used as an ingredient of moistureproof agents or hot-melt adhesives. The recent increasing awareness of environmental issues has boosted the demand for wax-containing materials to have biodegradability. There are various biodegradable waxes. Having extremely low melt viscosities, waxes are usually used as mixed with resins and the like.
For example, a biodegradable hot-melt adhesive composition mainly comprising a thermoplastic resin and a tackifier is known (see JP-A-5-339557). In the composition disclosed, at least one of the thermoplastic resin and the tackifier is polylactic acid or a lactic acid copolymer, and the composition contains 50% by weight or less of a wax. A biodegradable hot-melt adhesive composition comprising raw rosin, natural rubber, and a wax is also known (see JP-A-7-278510). These compositions do not have sufficient moistureproofness. Therefore, a film prepared by melt molding the composition is unfit for use as a liner of packaging materials and containers required to have moistureproofness. While the compositions have an increased melt viscosity over that of the wax used therein in view of nature of the use as a hot-melt adhesive, their melt viscosities are still not so high as to enable melt molding to produce T-die film or blown film by use of an extruder.
Among melt moldable, biodegradable compositions is the one disclosed in JP-A-2001-288295, which contains corn gluten meal and natural rubber as main components and is described as having high water resistance and high wet strength as well as biodegradability. The composition is, however, insufficient in moistureproofing performance similarly to the hot-melt adhesive compositions disclosed in JP-A-7-278510 and JP-A-59-66598 supra.
JP-A-2002-266284 discloses a coating composition providing a moistureproof coat. The composition is prepared by mixing natural rubber and natural wax using a solvent. However, the coating method using a solvent involves the smell and safety problem caused by a residual solvent in an intermediate or final product. To use a large quantity of a solvent also gives adverse influences on the natural environment and the working environment.
Hence, there has been no composition available that is melt moldable, sufficiently highly moistureproof, and biodegradable, nor a composition that is highly adhesive, sufficiently highly moistureproof, and biodegradable.